1. Field of the Invention
The invention generally concerns a method and a device for determining leaks in sewage pipes, and more particularly provides a radially segmented sensor device placed inside the pipe where leakage current determines location and volume of a leak.
2. Discussion of Related Art
A method and a device of this general type have become known from European Patent 0 786 653. This known technology involves a method using a measuring probe to determine the location and the size of leaks in sewers and similar conduit systems where a pipe exists which is not too highly conductive electrically and laid in an electrically conductive underground.
Another method and a device of this type are also described in German patent 40 10 611. With this well-known device, a probe, which is designed to move in a longitudinal direction along the pipe, is fed into the sewer. Current paths emanate in all directions from the electrode, but basically extend in the direction of the sewage flow. Once the electrode is moved past a leak in the pipe, the electrical emission near the leak will increase, which can be measured from outside the pipe by an appropriate measuring system. This measuring system basically consists of a voltage source and a current measuring instrument, with the voltage source being grounded by means of a remote ground contact. A leak is characterized by the sewage seeping out into the surrounding soil, causing the flow of current on the electrode to change once this electrode is brought close to the leak.
One disadvantage with the known procedure, however, is that it responds less selectively to a leak of this kind. It is disadvantageous, in fact, that a relatively high current should flow along the sewer over the sewage itself, with the result that this method is not very selective.
With European Patent 0 786 653, a method and a device of the initially specified type was developed so that measurements can be made with substantially greater sensitivity, with the result that even smaller leaks can be determined in sewage pipes and the like. An essential feature is that no longer is a current-emitting electrode simply moved in a sewer, but rather that a measuring probe is moved in the sewer. The measuring probe basically consists of an electrically conductive mount on which at least one current flow sensor is arranged. The mount can be rigidly designed as a rod. It is possible, however, to use a wire spiral or a wire stretched in the pipe. The use of a rod allows it to be designed in one or more parts.
The prior art probe can be used not only to detect leaks in sewers, but in any kind of pipe system which assumes that an electrically conductive medium is guided in the conduit of pipes and the conduit itself is not too highly conductive (metallically), so that the current flow in the leak-free pipe wall is not too high in the direction of the surrounding earth.
Accordingly, the probe is not merely limited to pipes in sewers, but could also be used for detecting leaks with other conductive media in pipe conduits of corresponding conductivity. It is based on the effect of moving a conductive metal rod or metal-coated plastic rod or the like in a longitudinal direction inside a pipe, whereby a centric arrangement of the probe in the pipe is not inevitably assumed, but rather the probe could also be moved eccentrically in the pipe in a longitudinal direction.
It is also possible for several probes to be moved radially on the inside of the pipe wall on an equally appropriate mount. This would enable the radial position of the leak to be determined in addition to determining the axial position.
The measuring probes may consist of simple, current emitting electrodes whose discharge longitudinally, compared with the ratios in German patent 40 10 622, is now reduced by the factor 1/n (n=number of measuring probes). This arrangement signifies a very considerable improvement in the resolution vis-a-vis the above German patent.
One embodiment of the prior art probe consists of a conductive rod, on which two current sensors are arranged at a distance from each other, and the rod is connected on one side to a voltage source. The voltage source is connected in turn between the rod and a remote ground. The flow of current through the two current sensors is recorded and transmitted via a cable to the outside of the pipe, where a corresponding measuring system is arranged on the earth's surface.
The applicable principles of operation and structure of the known devices are set out below. If one introduces a conductive metal rod, supplied with a current, axially into a pipe filled with conductive fluid, then lines of current will extend from the surface of the metal rod through this fluid in the direction of the pipe wall. It has already been explained on the basis of the current state of the art that unwanted longitudinal currents may arise if no precautions are taken to achieve a focusing of the lines of current.
With European patent 0 786 653, a focusing of the lines of current at the probe was achieved by various devices through extending the relevant lines of current normally toward the wall of the pipe. These lines of current are of particular interest because they are highly sensitive to any change in the conductivity in the pipe wall, especially where a leak is found.
Hence, a method and a device were developed with which the lines of current which are normally found toward the pipe wall can be evaluated selectively while the other lines of current which extend at an angle or even longitudinally to the medium can be blanked out.
Provision was made for two current sensors to be arranged at a distance on a conductive metal rod and that these current sensors should each consist of a toroid core, which is enclosed in a winding, with the winding's connections leading outwardly. This involves a Rogowski coil. Any known current sensors can be used that are suitable for measuring relatively small currents within the range of few milliamperes.
What is important is that, if one takes the difference between the flow of current through one current sensor and the flow of current through the other sensor, a measuring current, or rather leakage current, is obtained which contains the information about the location of the leak. The size of the measuring current caused by the leak provides the information on the extent of the leak as long as the magnitude of the measured leakage current remains smaller than the known value of the fault current for one path through the pipe wall.
The probe is actually divided in three and consists of a rear section, a central section and a front section. The tripartite division is achieved in the first embodiment by the probe being constructed as a continuous metal rod, with the central section being defined, however, by the space that results from the two current sensors being arranged at a distance from each other. The central section is the sensor section, therefore. The current that is emitted here in the direction of the pipe wall is measured. The two outer parts of the probe only have the task of ensuring that the current of the central section flows in the direction of the pipe wall. This occurs through all three sections of the probe lying at the same potential plane, whereby all three sections in the first design example consist of a continuous metal rod or an equivalent plastic component which is metal-coated.
The requirement for the measurement is that the two current sensors should be constructed identically to allow them to make a statement as to the differential current.
In a further development the plan was to use only one current sensor, but through which two currents are passed, in order to obtain a difference between two currents. Here the measurement of the second current is not obtained by a second sensor arranged away from the first sensor, but rather by carrying out a mechanical separation in the continuous metal probe and connecting a lead to both the front of one of the adjoining probe sections and to the front of the other near the point of interruption. This lead is passed through the first current sensor causing a measuring signal to be produced at once in the first current sensor, which corresponds to the differential current being measured. In this way there is no longer the need to ensure that two identical current sensors are used since only one current sensor is required.
In a third embodiment the idea was not to design the probe as a metal rod, but rather as a relatively thin metallically conductive spiral, which can be moved along as a wire in the conduit so that even very thin sewage pipes (house mains, etc.) can be examined.
To improve the probe's quality there is the possibility of attaching two spherical electrodes or other conductive bodies, connected in a conductive manner with the wire at intervals to each other on this spiral wire. In the space between the two electrodes, the two current sensors described above are again arranged, which work according to the previously described principle. There is therefore no need of a metal rod of a length, say, of one meter, but instead a continuous spiral can be used, its range being defined by the probe.
In a fourth embodiment of the prior art the idea was that a stationary wire should be stretched in the pipe being examined which, instead of moving, has a movable probe attached to it. A voltage is then applied to the wire, and on this wire is then moved the probe which consists of the two current sensors referred to above, which are arranged at a distance from each other and are kept at a distance through appropriate rods. These rods are insulating.
The arrangement of the two current sensors at a distance from each other ensures that the two current sensors only record the section of the wire found between the two current sensors as the measured distance.
In another embodiment the equalized potential of the three probe sections, unavoidable with the metal rod with mechanical and electrical separation, can be produced by an active control switch. In this instance the current emitted through the central probe section could then be simply measured directly.
A disadvantage of the familiar measuring probe is that only an inadequate resolution of the measurement is given of the sewage pipe because the familiar measuring probe takes the measurement on one full circle of the pipe and accordingly fails to produce an exact measurement because the pipe wall can only be recorded as a totality. If a leak is present in the pipe wall, then a fault current of the measuring probe will flow through this leak. Overlying this fault current is the total leakage current extending over the circumference of the pipe along the pipe wall and thereby falsifying the measurement finding. This is the reason why, with the familiar measuring probe, there is the disadvantage that faults are hard to recognize because the leakage current extending along the pipe wall is relatively large compared to the fault current actually being recorded, and therefore the measurement is difficult to analyze.
There is also the further disadvantage that a location of the leak (above/below, on the side) is not possible because the measuring probe revealed in the publication referred to does not take a radial reading, but only an axial reading. Thus the false location of the leak in the pipe wall cannot be precisely determined from the circumference.